When someone lists the most famous gemstones such as diamond, topaz, aquamarine, emerald and garnet,
corundum does not usually get mentioned.
However, its two varieties are sure to be on any list of gemstones.
The red variety of corundum is known as ruby and all the other colors of corundum are known as sapphire.
The rare colorless or white crystals are called corundum in the mineral world,
but artificial colorless plates of corundum are called sapphire and used as
watch faces for high-end timepieces. That's because a sapphire crystal sounds
much more valuable than a corundum crystal.

Corundum is the third hardest natural mineral known to science.
The hardest mineral, diamond is still four times harder than corundum.
The second hardest is
Moissanite (Silicon Carbide) at 9.25 barely beats corundum's hardness of
9.0. The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds.
These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements.

PHYSICAL CHARACTERISTICS:

Color is highly variable.
The color can be white or colorless, blue, red, yellow, green, brown, purple, and pink; there are also instances of color zonation.

Luster is vitreous to adamantine.

Transparency: Crystals are transparent to translucent.

Crystal System is trigonal; bar 3 2/m

Crystal Habits include sapphire's typical six-sided barrel shape that may taper into a pyramid, and ruby's hexagonal prisms and blades.

Cleavage is absent, although there is parting which occurs in three directions.

Fracture is conchoidal.

Hardness is 9

Specific Gravity is approximately 4.0+ (above average for translucent minerals)